Chordates

Introduction to the phylum Chordata

A Characteristic of Many (but not all) Chordates: a backbone

3: Notochord: flexible rod between gut and nerve chord. This disappears in vertebrates as it is replaced by a spinal cord. The remnant is the intervertebral disks (show on skeleton)2. Dorsal, hollow nerve cord: Develops from ectoderm that forms a ridge, then a tube. Brain is at 1.7. Pharyngeal gill slits: upper digestive tube (pharynx, 6), after the mouth (8), opens to the outside of the body. Used for filter feeding in invertebrate chordates, and for respiration in fish. Absorbed after embryonic stage in amphibians, reptiles, birds, and mammals.4. Muscular, post-anal tail.

Chordate characteristics

Chordate Characteristics: Checking for understanding

Chordate Evolution:take out your fill in the blank sheets

A. chordatesB. VertebratesC. TetrapodsD. amniotes

What makes us chordates?Q: What are the four key characteristics that define the phylum chordata?

NotochordA stiff, flexible rod that provides support for the nerve cord.

In most vertebrates it takes the form of the intervertebral discs.

Dorsal, Hollow Nerve CordRemember the Ectoderm and neurulation?

A hollow tube that develops into the brain and spinal cord.

Pharyngeal Gill SlitsPortion of digestive tube that allows water to filter through without going through entire system.

Become portions of our jaw.

Postanal TailUsed for movement, balance in various species.

Chordate is not synonymous with vertebrateIts possible to be an invertebrate chordate.

The earliest chordates were actually invertebrates, and three groups of invertebrate chordates survive to this day. These include the Cephalochordates (lancelets)Urochordates (tunicates)

Not all chordates are vertebrates!!!Urochordata (tunicates)No notochord, nerve cord or tail in the adult form. Only pharyngeal slits.But ALL four are present in the larval form.

The lancelet Branchiostoma, a cephalochordate (f)Exclusively marineFilter feed using a mucus net across the pharyngeal slitsSegmented muscles develop from somites.

Vertebrata (a subphylum)In addition to the four characteristics discussed earlier, vertebrates display several other specific traits:CephalizationVertebral columnClosed circulatory systemNeural crest

The first vertebrates to evolve from invertebrate chordates were the agnathans (jawless fish)Note that jawless does not equal mouthless

Agnathans Jawless VertebratesMost are extinct only survivors are lampreys and hagfish.

No paired appendages or only a single pairLampreys prey on other fishHagfish mainly scavengers

Living Agnathans:A sea lampreyfilter feeders in streams as larvae, and become fish parasites as adults.

Jawless fish underwent their own adaptive radiation

and also gave rise to the three classes of jawed fish

Jaws first evolved in the now extinct placoderms (j)Skeletal rods that supported the pharyngeal slits evolved hinges, leading to jaws.Why would this have been an advantage?

Other placoderm innovationsPaired finsSkeletal elements (such as what again?) in appendages (k)

ChondrichthyesCartilaginous skeletonVery sharp sensory systems:Ability to sense electric fields nearbyLateral line system (most species of fish have it) row of tiny sensors that detect water pressure changes (vibrations)

Chondrichthyes: Cartilaginous Fish

Chondrichthyes: Additional featuresMostly carnivorous. Some (largest ones) suspension feeders.Teeth evolved from scales on skinSenses (in addition to keen sight, smell, and hearing) includeElectric field detectorsLateral line systemFertilization is internal. Development can beOviparous (eggs laid externally and hatch outside mother)Ovoviparous (eggs are held inside the uterus but not nourished by mother. Young exit uterus after hatching.Viviparous (young are nourished by a placenta).

Some Shark MisconceptionsTRUE or false: sharks must swim to keep from sinkingTRUE OR FALSE: sharks will die if they stop swimming.Swimming keeps water flowing over gills, but many rest on bottom, and, oily liver contributes to buoyancy

Osteichthyes consists of 30,000 species of boney fish

Osteichthyes: key featuresBoney skeleton.Gills covered by operculum. Water passes from mouth to pharynx and out the gills.Mostly oviparousSwim bladder for buoyancyProbably evolved in fresh water. Swim bladder is a modified lung (innovation m)

Osteichthyes bony fishGills covered by operculum for protectionThis and muscles near gill chamber allow for breathing while not swimmingSwim bladder (air sac) provides for buoyancy

Most Osteichthyans are ray-finned: fins supported by slender rays

OsteichthyesMost are ray-finned, but there are several groups of lobe-finned fish. It is thought that lobe-finned fish with lungs gave rise to the first terrestrial vertebrates (amphibians)

Some interesting creatures

Lobe finned fishLobed fins evolved to allow fish to wade under water through vegetation, or waddle from one pool to the next

But these lobes were also a pre-adaptation for life on land

Lobe-finned fish with lungs gave rise to the first tetrapodsAcanthostega, a Devonian relative of tetrapods

Some fish, like this coelacanth (Latimeria), are lobe-finned: bones extend into their pectoral and pelvic fins

Once three environmental conditions were met, a vertebrate tetrapod could make the transition to life on land

Conditions for Life on LandBuild up of oxygen levels to where terrestrial life could be supportedBuild up of a protective ozone layer so that u.v. radiation wouldnt be deadly.Establishment of a terrestrial ecosystem, with plants being eaten by animals (insects or their ancestors), which could be eaten by the 1st terrestrial vertebrates.

Whats the problem with this statement: Lobe finned fish evolved limbs so that they could live on land.

What is the more likely and more complex truth?

Lobe finned fish evolved limbs so that they could stay in the water. Their successful journeys from pool to pool preadapted them for life on land.

AmphibiansStill dependent on aquatic environment for egg-laying and fertilizationLarval stage is gilled and fish-likeDuring metamorphosis gills are reabsorbed and lungs/legs grow

AmphibiansOviparous but eggs lack shellsMoist skin aids in respiration

Include Salamanders - tailsFrogs no tailsCaecilians no legs, burrowing

Amphibian Metamorphosis: The dual life of a frog (Rana temporaria)

Amphibians are only partially adapted to terrestrial lifeWater is needed for egg laying and fertilization. Their larval stage is gilled and fish-like. During metamorphosis, the gills are reabsorbed, and lungs and legs grow. Lungs are not very efficient, and moist surfaces of skin and mouth aid in gas exchange. All species are oviparous.

Amphibians consist of three major ordersUrodeles (salamanders and newts)Anurans (frogs)Apodans (caeacilians: legless, burrowing).

ReptiliaEmerged from amphibiansKey differences are:Amniotic egg with shellKeratinized skin keeps them dry

EctothermsInternal fertilization

Key Adaptations in ReptilesScales made of the protein keratin cover the skin, providing a waterproof barrier. With the skin no longer serving a respiratory function, the lungs became well developed. Amniote eggs able to survive in a dry, terrestrial environment. This necessitates internal fertilization.

Reptiles are completely adapted to terrestrial life

Some extant reptiles (other than birds)

Snakes, Turtles, and Lizards, oh my! And crocs

Birds emerged as an offshoot of the dinosaurs during the middle Mesozoic period, at least 150 mya.

Key Avian adaptations: wings, hollowed bones, endothermic metabolism, and feathers

A small sample of living birds

Diversity of form and function in bird feet

Mammals also evolved from reptiles. Mammalian fossils extend back 220 mya.First mammals were probably small, nocturnal, insectivores.

Key characteristics:HairEndothermsMammary Glands

There are three main groups of mammalsMonotremes (egg layers)Marsupials (pouched mammals)Eutherians or Placentals

MammalsThought to have evolved from therapsids small, insectivorous reptiles

Following Cretaceous extinctions, they experienced a radiation

MonotremesEgg-laying mammalsPlatypusesEchidnasMammary glands, but no nipples

MarsupialsPouched mammalsbirth occurs very early and development is completed in pouch

Generally found in autstralia, though thought to have originated in North America.

Placental (Eutherian) MammalsTissue joining mother to embryo that remains until birth.

Key innovations in Chordate Evolution:G. vertebral columnI. jawsK. paired appendages with rays or bonesM. lungs or lung derivatives (like swim bladders)O. limbsQ. amniote eggU. hair, mammary glands S. feathers

Classification of Vertebrates

FishAquaticPharyngeal slits have gills and are used for respiration (and, rarely, filter feeding)AgnathaPlacodermsChondrichthyesOsteichthyesTetrapodsterrestrialPharyngeal slits are reabsorbed after embryonic period.AmphibiansReptilesBirdsMammals